1. Field of the Invention
The present invention relates to an imaging apparatus, an imaging method, and a program that can selectively execute an imaging operation in a normal imaging mode that outputs one exposure image signal in a unit period, and an imaging operation in a combined imaging mode that, in the unit period, outputs a long-time-exposure image signal corresponding to a relatively long exposure time and a short-time-exposure image signal corresponding to a relatively short exposure time.
2. Description of the Related Art
In an imaging apparatus of the related art in which a solid-state imaging device such as a charge-coupled device is used, the amount of light (exposure) input to the imaging device is adjusted by controlling an aperture and an electronic shutter speed. In other words, the amount of exposure is controlled. When imaging is performed on a bright scene, the amount of exposure is reduced so that overexposure is prevented from occurring due to saturation of an output signal of the imaging device. Conversely, for a dark scene, the amount of light is increased so that so-called underexposure is prevented from occurring.
In the case of performing imaging on a scene having a large difference in brightness, such as back-lit imaging and indoor-outdoor simultaneous imaging, insufficiency of a dynamic range of a solid-state imaging device may cause a problem in that only controlling the amount of exposure makes it difficult to appropriately reproduce bright portions and dark portions since the bright portions tend to be saturated and therefore overexposed and the dark portions tend to be underexposed.
To solve the problem, a method is disclosed in Japanese Unexamined Patent Application Publication No. 6-141229 and corresponding U.S. Pat. No. 5,455,621 in which, by using two different electronic shutter speeds in a field, or changing an electronic shutter speed in each field, imaging on bright area information and imaging on dark area information are separately performed, and the resultant information is processed into an image.
In addition, as an apparatus to which the above method is applied, there is an apparatus (wide dynamic range camera) that can perform imaging on an image having a wide dynamic range, and an apparatus and method that improve the quality of a combined image are disclosed in Japanese Unexamined Patent Application Publication Nos. 2002-84449 and 2004-120205 (corresponding United States Patent Application No. 2006-0033823A1).
In general, an operating state of a wide dynamic range camera is one of two types: a combined imaging mode in which wide dynamic range imaging is performed, and a normal imaging mode in which wide dynamic range imaging is not performed. In exposure control of a wide dynamic range camera of the related art, the combined imaging mode and the normal imaging mode have a common output amplitude (operation region) for an imaging device. As shown in FIG. 8, the operation region is set at a level lower than a saturation region of the imaging device. This is because, when the operation region of the imaging device extends to the saturation region, as shown in FIG. 9, saturation unevenness is generated due to a variation of a dynamic range for each pixel for a high luminance signal, and image quality in a high luminance portion deteriorates.
However, as described above, when the operation region is set at a level lower than the saturation region, the saturation unevenness is prevented from being generated. Nevertheless, a new problem occurs in that lowering of the operation region causes a deterioration in a signal-to-noise ratio.
In addition, it is expected that, in a normal operation mode, an operation region be extended. Accordingly, signal processing is necessary in which the saturation unevenness is reduced so as to be as small as possible.
An imaging apparatus is disclosed in Japanese Unexamined Patent Application Publication No. 7-240859 in which fixed form noise that is generated in a white portion due to a variation in a dynamic range for each pixel is prevented from being emphasized by aperture correction and which is configured to obtain a wide dynamic range.
By using this technology in signal processing in the normal imaging mode, the operation region can be extended and the signal-to-noise ratio can be increased.
Deterioration in the signal-to-noise ratio may be more problematic than the saturation unevenness depending on the purpose of a camera. In the normal operation mode, it may be expected that the operation region be extended, even if the above-described processing of reducing the saturation unevenness is not performed.